The Power of Streamlining

Aerodynamic streamlining is very important for people who want to ride fast.

•When on a conventional upright bike at twenty miles an hour on flat ground, something like 90% of cyclist effort goes to overcoming wind resistance.

•Vehicles with effective aerodynamic fairings can be significantly faster than unfaired vehicles in almost all conditions except long, steep climbs or in continuous stop-and-go traffic.

•The stronger you are and the faster you ride, the more aerodynamic features will help. In headwinds, aero helps everybody.

•As a rule of thumb; recumbent cycles with bottom brackets (pedal circle centers) higher than the seat base create an aerodynamic profile smaller than that of an upright road bike and define the class of fastest cycles.

•In hot weather a closed fairing may reduce body-cooling so much as to reduce the rider’s ability to exert themselves. Well ventilated fairings that shade the rider may improve performance even in hot, sunny weather.

Two main factors determine a cycle’s aerodynamic drag and thus its ultimate speed ability: 1) frontal area and 2) coefficient of drag. Bikes which lay the rider back present a smaller frontal area “to the wind”. A rider with loose flapping clothes on a bike with large flat surfaces facing across the direction of travel will have higher aerodynamic drag than a rider in tight slick clothing on a cycle with rounded “aero” tubing, and a much, much higher coefficient of drag than a rider enclosed in a sleekly-rounded hard fairing.

Streamlined vehicles are generally heavier, and will lose much of their speed advantage on rough or very hilly terrain, or in continual stop-and-go traffic.

The chart below presents data from 2002 testing by the Netherlands Human Powered Vehicle organization. Results are from a single rider using a power-output monitoring device, and all vehicles were tested in identical wind, grade and pavement surface conditions using similar high pressure tires. These results illustrate the difference in performance created by design for improved aerodynamics; the fastest and most efficient vehicles are those that have compact frontal areas and are streamlined. (This table is copied by permission of David “Legs” Larrington.)

Cycle Description

Speed at 250 Watt output (km/h)

Power required to go 40 km/h (Watts)

“Standard” bike – upright position

29.0

622

Racing bike – “on the tops”

32.0

469

Racing bike – “on the drops”

35.0

362

Triathlon bike

37.0

310

Thys Roeifiets – “high” seat

38.0

288

Recumbent bike – seat height 60 cm

38.5

277

Recumbent bike – seat height 40 cm

39.5

259

Recumbent bike – seat height 20 cm

41.0

234

Aluminium Alleweder trike

42.0

220

C-Alleweder trike

45.0

185

Recumbent bike – seat height 20 cm, with tail fairing

45.0

180

Recumbent bike – seat height 20 cm, with foam full fairing

51.0

135

Recumbent bike with full hard fairing

69.0

75

Streamlining permits fast riders to ride faster. Slow and leisurely riders will not benefit much in still air, but will in headwinds. This benefit is created because drag increases as the square of your (wind) speed; move twice as fast and wind resistance will become four times as great. Even at 15 miles an hour, aerodynamic drag is the dominant resistance that a cyclist must overcome; at 20 mph, aero drag is over 90% of resistance.

A body or shell around a cycle can potentially be designed to provide weather protection, crash protection, theft deterrence, increased visibility and aesthetic styling, in addition to aerodynamic benefit. These benefits may be enjoyed by all riders.